The present invention relates to the induction melting of reactive metals and alloys in water cooled metal crucibles.
As is set forth in G. H. Schippereit, et al, "Cold Crucible Induction Melting of Reactive Metals," Journal of Metals, February 1961, pages 140-143 and in Schippereit's U.S. Pat. No. 3,223,519 of Dec. 14, 1965, the art for some time has recognized the theoretical desirability of utilizing induction heating methods for the melting of reactive metals such as titanium, as a replacement for known industrial scale melting processes based up, e.g., consumable electrode arc melting techniques. In induction melting, an electric current is induced into the metal to be melted. Thus, by supplying an alternating current to a primary induction coil, a reverse alternating current is induced into any electrical conductor lying within the magnetic field of the coil and produces heating in the conductor.
In induction melting, a crucible is needed to contain the molten pool which forms from the metallic charge which lies within the magnetic field of the coil. In most induction melting processes, the crucible is formed of a refractory material such as aluminum oxide. However, reactive metals, including titanium, zirconium, hafnium, molybdenum, chromium, niobium, and other metals and alloys of this type cannot be melted successfully in refractory crucibles. When molten, these metals react with and dissolve the crucibles causing the molten metal to become contaminated. This problem is avoided in a cold-mold arc-melting furnace because the crucible, usually copper, is cooled to avoid temperature sufficiently high for reaction to occur with the enclosed reactive metal. Copper is preferred for such cold crucibles to provide good thermal and electrical conductivity and thermal shock resistance. However, early attempts to induction melt reactive metals in water-cooled metal crucibles were unsuccessful. In such cases, the primary coil surrounding the crucible induced strong electrical currents in the crucible which led to insufficient power transfer to the metal charge held within the crucible to initiate melting.
In response to these problems, Schippereit proposed a low frequency, coreless induction melting process in which the metal charge is held in a metal crucible which is "segmented", i.e., constructed of metal segments electrically insulated from one another by a non-conductive material (e.g., thin plastic, ceramic). In this way induced currents generated from a surrounding induction coil could not flow in a continuous manner circumferentially around the crucible, thereby minimizing attentuation of the magnetic flux intended for establishing induction currents in, and melting, the charge held by the crucible, and preventing damage to the crucible and/or alloying of crucible metal with charge metal.
A large effort was made to reduce the segmented cold-wall induction melting concept proposed by Schippereit to commercial practice for melting reactive metals including titanium. This work is reported in "Induction Heating Process for Melting Titanium", MLTDR 64-209, July 1964, Contract AF 33(600)-39039. The process was generally unsuccessful and was abandoned.
In U.S. Pat. No. 3,775,091 to Clites, et al of Nov. 27, 1973, it was pointed out that the induction process and apparatus of Schippereit, while successful on small scale equipment, could not be scaled up for use in large crucibles. According to Clites, et al, as the metal charge melted in the Schippereit process, molten metal filled the interruptions in the crucible segments, thus shorting the segments together and returning to the undesirable situation where induction currents were established in the crucible.
The solution to this problem as proposed by Clites, et al, was to generally retain the segmented crucible concept but to use a slag or fluxing agent in association with the metal charge in order to produce a self-generating and self-renewing insulting material between the crucible segments and provide a liner for the interior crucible surface. In addition to changing the melt process advocated by Schippereit, Clites later modified the crucible design itself. In Clite's later crucible design, the side segments of the crucible were no longer electrically insulated from each other but rather were electrically connected at the base of the crucible. Besides U.S. Pat. No. 3,775,091, other descriptions of this "inductoslag" melting or casting process may be found in U.S. Pat. No. 4,058,668 and in P. G. Clites et al, "Preparation of Ingots and Shaped Castings by Inductoslag Melting", Proceedings of the Fifth International Symposium on Electroslag and Other Special Melting Technologies, Oct. 16-18, 1974; Bulletin 673, "The Inductoslag Melting Process," U.S. Department of the Interior, Bureau of Mines (1982); and Bureau of Mines, Report of Investigations, RI 7268, "Inductoslag Melting of Titanium" (1969), all of which are incorporated by reference herein.
While the "inductoslag" melting process developed by Clites has been discussed in the prior art as having at least theoretical applicability to the melting of a variety of reactive metals and their alloys, our work with this system encountered significant problems. First, the use of slag in melting reactive metals has never been commercially accepted due to the concern for property degradation as a result of slag contamination of the metal. Second, melting and casting under the preferred conditions for "inductoslag" processing--i.e., under a partial pressure of argon or helium--did not produce castings of satisfactory quality for commercial purposes. By conducting the process under vacuum we were able to eliminate the casting quality shortcomings of the inert gas system. However, operation under vacuum introduced new problems not previously encountered or mentioned in the literature. Specifically, the slag underwent vaporization from the melt under the vacuum levels needed to produce quality castings. The vaporization formed extensive deposits throughout the melt furnace, and these deposits severely contaminated the vacuum system, including the vacuum pumps. Under certain conditions these deposits reacted with the molten metal and caused extensive gas porosity in castings. Under all conditions, the vaporization of slag extracted heat from the molten metal bath and significantly reduced the amount of metal that could be poured from the crucible into the casting mold.